1. Field of the Invention
This invention relates to the field of gas purification, and more specifically to the removal of trace impurities from inert gases such as nitrogen, helium, and argon using solid scavenger adsorption materials. More particularly, this invention comprises a method of reducing concentrations of trace impurities, such as hydrocarbons, carbon monoxide, and carbon dioxide, from process gases to parts-per-billion and sub-parts-per-billion levels using an ultra-low emission carbon based scavenger. This invention further relates to reducing concentrations of impurities such as organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons, carbon monoxide, and carbon dioxide, as well as other impurities such as oxygen and larger quantities of moisture (H2O), from process gases by combining an ultra-low emission carbon material of this invention and a second scavenger material capable of removing oxygen and/or moisture from the process gas.
2. Description of the Prior Art
Numerous products and processes require pure gases. One known method of gas purification involves the adsorption of process gas impurities on a bed or column of solid scavenger material. In these solid adsorption methods, impurities are caught by the surface of the scavenger material while the process gas preferably passes unaltered through the bed or column.
Commonly used solid scavenger adsorption materials include alumina, activated carbon, silica, adsorption clays, and secondary scavengers. Activated carbon, for example, is used in PSA (Pressure Swing Adsorption) plants and for solvent recovery from air in painting facilities (See, for example, Wood and Stampfer, Carbon, 30:593 (1992); Wood and Stampfer, Carbon, 31:195 (1993); Nelson et al., Am. Ind. Hyg. Assoc. J., 33:797 (1972); and Nelson et al., Am. Ind. Hyg. Assoc. J., 52:235 (1991)). These techniques are known to reduce selected impurities in a gas stream down to single digit percentages, and perhaps even as low as ppm (parts per million) concentrations. However, the use of solid scavenger adsorption materials operating at ambient conditions to reduce parts-per-billion (ppb) levels of impurities, particularly hydrocarbons, to sub-ppb levels without contaminating the gas stream with other impurities such as moisture is not known.
For most applications, reducing impurities in gases down to the ppm level is satisfactory. However, ultra-pure gases having impurity concentrations not exceeding ppt (parts-per-trillion (ppt) levels are required in a growing number of industries. For example, in semiconductor fabrication processes, gases such as nitrogen, helium and argon are often required to not have more than low ppb or sub-ppb impurity levels to ensure that the impurities do not degrade the quality, and hence the performance of the semiconductor chips. Gas purification systems are therefore widely used in the manufacture of semiconductors to remove process gas impurities to very low, trace concentrations.
The desire to develop methods to reduce impurities in process gases down to sub-ppb concentrations is further driven by the present ability to measure impurities at extremely low levels. Modem chemical instrumentation such as Atmospheric Pressure Ion Mass Spectrometry (APIMS) permits the detection of process gas impurities such as carbon monoxide, carbon dioxide, oxygen, and moisture (H2O) at sub-ppb concentrations.
The advances in the detection of trace levels of hydrocarbons with APIMS has motivated researchers to further reduce the levels of these impurities in ultra-pure process gases to below the limits of detection of this supersensitive instrumentation. One challenge has been to develop gas purification materials and techniques that remove hydrocarbon impurities from an ultra-pure gas without adding trace amounts of other impurities.
Conventionally activated carbon, for example, is known as a very effective adsorbent for removing hydrocarbon impurities from gases. However, conventionally activated carbon is typically activated at 200xc2x0 C. to 400xc2x0 C. in gas streams contaminated with ppm levels of impurities such as moisture and CO2. After conventional activation, the carbon material contains trace amounts of water and CO2 that are either not completely removed during activation or re-adsorbed in the contaminated environment of the treatment process. The carbon material may also produce trace amounts of moisture and CO2 during thermal activation due to chemical reaction of residual functional groups or adsorbed species, such as by dehydroxylation or decarboxylation reactions. The residual water and CO2 in the conventionally activated carbon material are then released in small quantities into a gas stream during a gas purification process, thereby causing significant contamination of the gas and rendering the effluent gas useless for high purity applications. In some cases, conventionally activated carbon is characterized as xe2x80x9chydrophobicxe2x80x9d (repels or fails to adsorb water), even though traditionally activated carbon has been shown to weakly adsorb moisture upon exposure of a gas containing several hundreds to several thousands of ppm of moisture (see, for example, Barton et al., Carbon, 22:22 (1984), which is specifically incorporated herein by reference). However, this adsorbed moisture, is also easily released into a process gas stream during purification of the gas. Thus, reducing hydrocarbon impurities in a process gas to sub-ppb levels while maintaining very low levels of water vapor and CO2 has proven extremely difficult.
Accordingly, it is an object of this invention to provide a method for reducing the concentration of hydrocarbon impurities as well as other impurities in a process gas to sub-parts-per-billion (sub-ppb) levels, while at the same time not emitting higher levels of other contaminants, such as water vapor and CO2, into the process gas being purified.
Another object of this invention is to provide xe2x80x9cultra-low emissionxe2x80x9d (ULE) carbon materials for reducing trace impurities such as organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons, carbon monoxide (CO), carbon dioxide (CO2), and small amounts water vapor from process gas streams such as helium (He), nitrogen (N2) and argon (Ar) to parts-per-billion (ppb) and sub-parts-per-billion (sub-ppb) levels.
Another object of this invention is to provide a method of producing ultra-low emission (ULE) carbon materials capable of reducing the concentration of organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons, carbon monoxide (CO), carbon dioxide (CO2), and water vapor (H2O) from a process gas to ppb and sub-ppb levels.
It is a further object of the present invention to provide a method of purifying gases with ultra-low emission (ULE) carbon materials prepared according to the method of this invention, wherein the method reduces trace amounts of hydrocarbon, carbon monoxide (CO), carbon dioxide (CO2), and water vapor (H2O) impurities to ppb and sub-ppb levels.
It is a further object of the present invention to provide a one-component gas purifier system comprising a bed of an ultra-low emission (ULE) carbon material of this invention capable of reducing trace levels of organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons, carbon monoxide, carbon dioxide, oxygen and water vapor from a process gas to ppb and sub-ppb levels.
Yet another object of this invention is to provide a two-component gas purifier system comprising an ultra-low emission (ULE) carbon material of this invention and a secondary scavenger material to remove impurities such as oxygen and larger quantities of moisture that are not scavenged by the ultra-low emission (ULE)carbon material. The two-component purifier system of this invention acts as a combination gas purifier capable of producing a purified gas with only sub-ppb levels of impurities such as organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons, CO, CO2, O2, and water vapor. In one embodiment, the secondary purifier is a chemical scavenger material that removes a variety of impurities such as moisture and oxygen from gas streams but does not remove hydrocarbons. In one embodiment, the secondary purifier is an organometallic resin as disclosed in U.S. Pat. No. 4,603,148, which is specifically incorporated herein by reference. Other materials that are suitable for use as secondary purifiers for the removal of moisture include, but are not limited to, inorganic high-surface-area solids such as oxides and mixed oxides, e.g., alumina, silica, silica-alumina, aluminosilicate zeolites and other molecular sieves. These materials may be modified by salts, oxides or hydroxides of the Group IA or IIA metals, and preferably are thermally activated, as described in copending U.S. Provisional Patent Application No. 60/251,000, filed Dec. 4, 2000, which is specifically incorporated herein by reference. The secondary purifier material is referred to herein as a xe2x80x9csecondary scavengerxe2x80x9d.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described therein, one embodiment of this invention provides a method for producing an ultra-low emission (ULE) carbon material, comprising heating a carbon material under inert conditions at a temperature and for a time sufficient to remove substantially all of the water and carbon dioxide (CO2) contained in the carbon material to produce an ultra-low emission (ULE) carbon material, and transferring the ultra-low emission (ULE) carbon material to a container under conditions that do not allow moisture, carbon dioxide, or other atmospheric contaminants to be reintroduced into the ultra-low emission (ULE) carbon material.
To further achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described therein, another embodiment of this invention comprises a method for removing impurities such as organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons, small amounts of water vapor, carbon monoxide (CO), and carbon dioxide (CO2) from process gases, the method comprising contacting a process gas with a one-component gas purifying system comprising an ultra-low emission (ULE) carbon material produced according to this invention, wherein the concentrations of the trace impurities in the process gas are reduced to below about one part-per-million (ppm), and preferably to below about one part-per-billion (ppb) upon contacting the process gas with the ultra-low emission (ULE) carbon material of this invention.
To further achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described therein, another embodiment of this invention comprises a two-component gas purifying system comprising a canister containing ULE carbon material of this invention connected in series with, and downstream of, a canister that contains a bed of a secondary scavenger capable of removing larger amounts of moisture and carbon dioxide from a process gas, the series-connected canisters forming a two-component gas purifier that operates to purify an input gas stream.
To further achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described therein, another embodiment of this invention comprises a two-component gas purifier system comprising a canister having an upstream portion that contains a bed of secondary scavenger material capable of removing larger amounts of moisture and carbon dioxide from a process gas, and a downstream portion that contains a bed of ultra-low emission (ULE) carbon material of this invention.
To further achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described therein, another embodiment of this invention comprises a two-component gas purifier system comprising a canister containing a secondary scavenger material intermixed with an ultra-low emission (ULE) carbon material of the present invention.
Additional objects, advantages and novel features of this invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods particularly pointed out in the appended claims.